312 jMr Harris on the UlU'dy of fixing 



derived, Avhich may be shewn thus : — If a metallic wire be fixed 

 through the bulb of an air thermometer, and an electrical dis- 

 charge be transmitted through the wire, the rise of the fluid 

 will measure the heat evolved. Let the same wire be now 

 passed through a draw-plate vmtil it be drawn into four times 

 the length. Let this wire be divided into four parts, and fixed 

 in the thermometer as before ; on passing a similar charge, the 

 four wires will evolve the same heat as the original mass. A 

 similar result will be obtained if the original wire be flattened 

 by passing it between rollers, so as to expand it into a surface. 

 If the quantity of metal be present, therefore, it is of no conse- 

 quence as to the form under which we place it; — it cannot be 

 supposed that by rolling a metallic surface into a dense cylin- 

 drical form, we thereby make its conducting ^ower greater than 

 it was before; consequently we do not diminish it, when, on the 

 contrary, we expand it into a surface. 



35. It would seem, however, that if any advantage is to be 

 obtained by form, it is on the side of the superficial conductor. 

 Sir H. Davy found that the conducting power of a metal was 

 improved by exposing it to a cooling medium : now, in expand- 

 ing a mass of metal into a large surface, we expose it to a 

 greater extent of air, by which the heating effect of a discharge 

 is much diminished ; so that a quantity of metal formed into a 

 hollow tube might possibly, from this cause, escape in some par- 

 ticular instances, when the same quantity, in thefoi'm of a small 

 rod, might be melted. It is highly probable that, in electrical 

 conduction, the electric matter operates first upon the surface, 

 and so on in parallel strata until it pervades the mass. If a ball 

 of wood be covered with one layer of silver or gold leaf, and a 

 charge be passed on it sufficient to destroy the metal, then, on 

 gilding the ball carefully with a double layer, we find that on 

 passing the same charge it will remain perfect, which shows that 

 the inner layer has transmitted some of the shock. If we sup- 

 pose the whole sphere to be made up of distinct layers in this 

 way, it is clear that the last will protect all the others, as in the 

 case of the surface of wires above mentioned ; the electric mat- 

 ter has evidently a tendency to pass first upon the outer stra- 

 tum, and then upon the next, and so on ; the next in succes- 

 sion, taking up the superabundant quantity with which the 



